This invention relates to a recording medium which includes a plurality of information recording layers and a recording apparatus and a recording method ready for a recording medium of the type mentioned.
Optical discs, optical cards and so forth are known as optical recording media on or from which information can be recorded or reproduced optically. A laser beam of a semiconductor laser or a like element is used as a light source and converged by a lens to form a light spot which is illuminated upon an optical recording medium of the type mentioned to record or reproduce information on or from the optical recording medium.
Development of a technique for further increasing the recording capacity of such optical recording media as described above has been and is being carried out energetically. Conventional effects to increase the information recording density on an optical disc have been directed principally to increase of the recording density on a recording surface of the disc. For example, it has been attempted to reduce the track pitch and/or raise the recording density in a linear velocity direction in recording and reading scanning, for example, in combination with a decrease in wavelength of a light source for emitting a recording beam or with signal processing of a reproduction system.
However, even if the wavelength of the light source is reduced, it is limited down to the ultraviolet region. Meanwhile, the pit size can be reduced to a level with which the pit can be transferred to a disc upon cutting. Therefore, as far as a two-dimensional region of a disc is utilized, the attempts to raise the recording density are limited soon.
Therefore, also a technique which makes use of a three-dimensional region is attempted to increase the capacity of a disc. In particular, attention is paid to a multi-layer disc wherein a plurality of information recording layers are layered in order to further increase the recording information density in the thicknesswise direction of a disc.
A multi-layer recording medium in which a plurality of recording layers are layered is characterized in that it can have a recording capacity increased in accordance with the number of recording layers and can be combined readily with other high density recording techniques. As one of such multi-layer recording discs, for example, a DVD-ROM (Digital Versatile Disc ROM) which is a reproduction-only optical disc has been placed into practical use.
Several documents such as U.S. Pat. Nos. 5,740,136, 5,754,507, 5,682,372,5,920,527, 6,061,310, 6,330,212 and 6,552,971 disclose techniques applied to a DVD-ROM or the like which includes two recording layers.
It is expected that not only multi-layer media of the ROM type but also recordable multi-layer recording media wherein a plurality of recording layers of a phase change material, a magneto-optical material or a dye material are layered can be placed into practical use in future. For example, as regards discs of the DVD type, it is estimated that multi-layer recording layers are implemented also with discs of the write-once type called DVD-R or DVD+R and discs of the rewritable type called DVD-RW or DVD+RW.
Incidentally, where a recording operation is performed for a multi-layer recording medium, movement of the recording point between layers is performed in the recording process. For example, the recording layers of a two-layer disc are possibly used in such a form that data recording is performed first for the first layer (layer 0) and then data recording is performed for the second layer (layer 1).
Where the multi-layer recording medium is a reproduction-only disc such as a DVD-ROM described above, since the amount of data recorded on the disc in advance is known and no user data are written on the disc any more, the folding back position of the reading point in movement between layers is fixed, and the folding back position may be recorded, for example, in management information in the lead-in area. More particularly, information of the last end position of the layer 0 may be recorded.
However, where discs which allow recording such as write-once type discs or rewritable type discs are considered, only if information of a fixed folding back position is recorded merely in the lead-in area or the like, this is insufficient for practical use.
For example, in a recordable disc, information of the lead-in area and so forth is not finally determined until after the entire disc is closed (or finalized) after data recording. In this instance, the folding back position information is not recorded until after disc closing is performed.
Further, when additional writing is performed in a multi-session system, since information of the lead-in area is written upon closing of the first session, actual folding back position information is not reflected. For example, if recording of the second session is performed after the first session is closed and the second session extends from the layer 0 to the layer 1, the actual folding back position information is not recorded as information of the lead-in area.
The situations described above give rise to a problem that such a situation that a disc recording and reproduction apparatus cannot actually grasp the folding back position accurately occurs and this deteriorates the compatibility and the feasibility in use of the disc.